Capacitors are passive elements that are extensively used in integrated circuits for storing an electrical charge. Capacitors, which include conductive plates separated by an insulator, have a capacitance contingent upon a number of parameters, such as plate area, intra-plate spacing, and the insulator's dielectric constant. Capacitors are found in filters, analog-to-digital converters, memory devices, control applications, and many other types of integrated circuits, and may be used for electrostatic discharge (ESD) protection.
One common type of capacitor found in integrated circuits is a vertical parallel plate capacitor (VPP). In particular, VPP capacitors may be integrated into an integrated circuit during back end of line (BEOL) processing forming the stacked metallization layers of multi-level interconnect structures or otherwise formed using BEOL process techniques and materials. Copper metallurgy, which has a higher conductivity than aluminum metallurgy, is used in lower metallization layers of the interconnect structure to increase signal propagation speed. However, aluminum metallurgy is preferred in upper metallization layers of the interconnect structure because of the recognized advantages of aluminum metallurgy over copper metallurgy for chip and package solder and wire bonding connection pads.
Consequently, the plates of VPP capacitors may have either an aluminum metallurgy or a copper metallurgy. As an artifact of the BEOL processing, the sidewalls and bottom of each copper plate is lined by a barrier layer. In contrast, aluminum plates are clad on only the top and bottom by barrier layers. Under ESD testing and during ESD events in an operating device, aluminum plates have been demonstrated to be more prone to failure than copper plates. Generally, ESD-promoted failure may occur by crack initiation and propagation in the dielectric material bordering the plate followed by melting and flow of the aluminum or copper from the plate into the crack. Because of the confinement by refractory metal cladding on the top and bottom surfaces, aluminum plates fail by a lateral cracking mechanism. In contrast, copper plates are confined by refractory metal cladding on the sidewalls and bottom surface and, consequently, fail in a vertical direction.
VPP capacitors are needed that exhibit increased resistance to ESD-promoted failures either during testing or during device operation.